Photovoltaic (“PV”) devices are used in solar energy systems for converting sunlight to electricity. The basic unit of a PV device is the PV cell, which produces an electrical output when illuminated. PV modules contain multiple PV cells electrically connected in series and/or parallel combinations to yield a desired output voltage and current, and are packaged in a suitable form for outdoor use. PV modules, also known as “solar panels,” typically form the basic unit of a solar energy generation system.
One of the fundamental characterization tests of a PV device is the measurement of its current versus voltage relationship, or “I-V” curve. FIG. 1 depicts exemplary I-V curves for a PV device, including curves under both illuminated and non-illuminated conditions (respectively, “light I-V” and “dark I-V”).
FIG. 1 depicts a graph with a voltage axis 100 and a current axis 102, representing voltage and current at the terminals of a PV device during measurement of an I-V curve. In FIG. 1 the polarity of the current axis 102 is defined as positive when positive current flows from the positive terminal of a testing system to the positive terminal of the PV device under test (“DUT”). With this definition, photocurrent is negative. The graph is conventionally divided into quadrants (104, 105, 106, 107) according to the polarity of voltage and current.
A light I-V curve 110 is measured when the PV device is illuminated and produces a photocurrent. In the power quadrant 104 the PV device sources power, which must be dissipated by the testing system. Isc (112) is the current at short-circuit (voltage V=0) and Voc (116) is the voltage at open-circuit (current I=0). The maximum power point (“MPP”) 114 occurs at a point where the power curve—the product of voltage and current, not shown—reaches a maximum. The light I-V curve 110 and the associated maximum power will be a function of the illumination intensity as well as the parameters of the PV device.
I-V measurements of illuminated devices in the power quadrant 104 comprise the majority of PV device testing. For these measurements, the testing system only sinks power provided by the DUT and does not need to supply power. However, valuable information can also be obtained from measurements in the other I-V quadrants, where the testing system must source power, including measurement of the light I-V curve 110 in the reverse-bias quadrant 105 and measurement of the dark I-V curve 120. These data can be used to determine various device parameters including shunt resistance, series resistance, diode saturation current, diode ideality factors, and other parameters. Therefore, it is often desirable to characterize PV devices using four-quadrant measurement systems, i.e. systems with the capability to both source and sink power. See, for example, D. L. King et al, “Dark Current-Voltage Measurements on Photovoltaic Modules as a Diagnostic or Manufacturing Tool,” IEEE PVSC, 1997, which is incorporated herein by reference. For dark I-V measurements, it is desirable to measure at voltages and currents up to ˜1.5-2× Voc or Isc, respectively, while also providing accurate current measurements all the way down to the microamp range, to allow proper analysis of data at low voltages).
For a PV cell, Voc is typically <1 V, and Isc is typically in the range of 1-10 A, depending on cell technology, size, and illumination intensity. PV modules contain many PV cells connected in series combinations, and therefore produce significantly higher voltages. For example, typical ˜250 W PV modules using silicon-based PV cells have Voc of 20-50 V and Isc of 5-10 A, while typical 50-150 W thin film PV modules using amorphous silicon, CdTe, or CIGS cells have Voc 50-150 V and Isc ˜1.0-2.5 A, although other ranges are also possible.
In addition to measuring the I-V curve of a PV device, it is often desirable to use an I-V testing system to maintain a PV device at a specific point along its I-V curve for an extended period. For example, a device may be held at Isc, MPP, Voc, or other points along the I-V curve during extended illumination (also known as “light soaking”). For such tests, the I-V measurement system operates in a continuous mode, rather than in a pulsed or transient mode.
Exemplary instruments providing four-quadrant I-V testing of PV cells and small modules include, for example, the Series 2400 SourceMeter® (a registered trademark of Keithly Instruments, Inc.) and related products, manufactured by Keithly Instruments, Inc. These instruments provide four-quadrant transient and continuous I-V measurement capability, but only over limited parameter ranges that exclude high-current cells and most typical PV modules. Multiple instruments may be used in series and/or parallel combinations to access higher current or voltage ranges. However, this approach is not economical, particularly when extended to measurement of many PV modules.
Other exemplary instruments include DC electronic load systems, available from a number of manufacturers, which are available over wide ranges of current and voltages suitable for PV modules. However, such systems measure only in the power quadrant 104. The units may be extended into the reverse-bias quadrant 105 by combination with a biasing power supply. However, this adds only limited additional functionality.
Another approach is to combine a bipolar (source/sink) power supply with a high-precision digital multimeter providing current and voltage measurement. However, such power supplies are typically constructed to provide any output current at any voltage, within the respective current and voltage ranges of the instrument, which leads to higher cost than required for the PV module testing application.
Typical existing commercial I-V measurement systems for PV devices have some or all of the shortcomings discussed above; therefore, there is a need for an improved I-V measurement system for PV modules.